Patent GB-390400-A (GB)
Abstract: 390,400. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.- (Assignees of Shore, H. ; 19, Pingry Place, Elizabeth, New Jersey, and Whitaker, J. N. ; 95, Highland Avenue, Bronxville Manor, Tuckahoe, New York, both in U.S.A.) Aug. 22, 1932, No. 23508. Convention date, Aug. 22, 1931. [Classes 40 (iii) and 40 (v).] Picture currents of constant frequency and varying amplitude are rectified and combined with a screen frequency to produce signals of constant amplitude and frequency but varying duration which are shaped so as to be of square wave form and used to key a carrier frequency. The screen frequency governs the frequency at which the signals of constant amplitude occur and therefore the detail of the reproduced picture. Light from the picture is interrupted by a chopper disc and falls on a photo-electric cell. The resulting oscillations are applied to terminals 33, 35, Fig. 1, amplified by the valve 49, rectified by the valves 69, 71 and taken from the potentiometer 85 to the potentiometer 25. A screen frequency governing the definition of the picture, is applied to terminals 1, 3, amplified by the valve 15 and fed to the potentiometer 25. The screenfrequency and picture signals are applied to the grid of valve 31, the picture signals controlling the period when the valve 31 is rendered conductive by the screen frequency. The plate circuit of valve 31 includes a square-wave amplifier comprising valves 105, 113. The output of valve 105 includes a resistance 109 connected in the grid circuit of valve 113, while the plate circuit of valve 113 includes a resistance 117 and is connected to the grid of valve 105 by a condenser 121. A heavy current in the plate circuit of valve 105 causes the potential on the grid 111 to be negative relative to its previous condition and renders valve 115 non-conductive, and when the current in the plate circuit 107, 109 decreases, the potential of grid 111 becomes relatively positive and the plate circuit 115, 117 takes current reducing the potential on the grid 103 and causing the current in resistance 109 to decrease more rapidly than normal. The resulting output across resistances 109 or 117 is substantially of square-wave form, and is applied to the centre of the transformer winding 139 to modulate the carrier frequency generated at 143. The modulated carrier frequency is taken from terminals 199, 201 for transmission, while monitoring may be accomplished from the transformer 183 and jack 203. The screen frequency applied to terminals 1, 3 is derived from a generator comprising a rotor 237, Fig. 4, and a stationary pick-up coil 247. The generator is driven by the scanning apparatus and preferably several generators are employed, each giving a different frequency, thus enabling the detail of the reproduced picture to be suitably chosen. A cam 267 driven by the scanning device closes contacts 269 for alternate lines of scanning, thus energizing relay 271 and reversing the phase of the screen frequency fed to the terminals 1, 3. This results in alternate lines of the reproduction being displaced, Fig. 3, b, (not shown), to produce a more detailed reproduction. At the receiver, Fig. 5, a plurality of aerials 275, 277, 279, geographically spaced to avoid fading effects, feed radiofrequency amplifiers RFA, detector circuits D, audio-frequency amplifiers AFA and further rectifier circuits R, the outputs of which are combined to feed a tone keyer 283, which feeds an amplifier 289, and a rectifier 29] to control the recording device 293.